Latexes are suspensions or emulsions of organic polymer particles in an aqueous medium. One challenge in the preparation of any synthetic latex is the production of a stable latex with minimal amounts of coagulum (polymer recovered in any form other than that of a stable latex). As noted in Emulsion Polymers and Emulsion Polymerization, "The Formation of Coagulum in Emulsion Polymerization," by J. W. Vanderhoff, 1981, American Chemical Society, coagulum is produced in all sizes of reactors and poses several problems. Such problems include loss of yield of the desired latex, processing difficulties due to the necessity of clean-up, more batch-to-batch variation in latex properties, and health, safety, and environmental problems insofar as the coagulum must be disposed of. Disposal is even more problematic due to entrapment within the coagulum of toxic monomers such as vinyl chloride and acrylonitrile. According to this reference, the formation of coagulum is due to either (1) a failure of the colloidal stability of the latex during or after polymerization, which leads to flocculation of the suspended polymer particles, or (2) polymerization of the monomer(s) by mechanisms other than by the intended emulsion polymerization.
In general, smaller polymer particles tend to result in more stable emulsions, and therefore, tend to form less coagulum. Aqueous polymerization of ethylenically unsaturated monomers in the absence of emulsifiers or surfactants leads to relatively large polymer particles, while the use of emulsifiers or surfactants leads to polymerization in small micelles, which produce relatively small polymer particles. Small particle sizes (those less than 100 nm) have typically been achieved by using relatively high levels of small molecule anionic surfactants such as AEROSOL-OT.TM. (sodium dioctyl sulfosuccinate) and AEROSOL NPES.TM.2030, (ammonium nonylphenoxy polyethoxy ethanol sulfate) which are both sold by Cytec Industries, Inc., West Paterson, N.J., U.S.A.
The particle size of a latex can also have a direct impact on the performance of a film or coating prepared from that latex. Among those properties that can be affected are gloss, clarity, film formation, and substrate penetration (for porous substrates). Overall, a smaller polymer particle size will have a positive effect on such properties. However, small particle size and the properties directly affected by it are not the only important properties in water-based coatings. Water-resistance/sensitivity of the final coating is also very important. A high level of anionic surfactant, the very component which promotes small particle size in the latex, and all of its concommitant advantages, is also likely to be detrimental to the water-resistance of the final coating.
Prior inventors have explored a modified approach, in which the same types of anionic groups utilized in the small molecule surfactants are bonded to polymers, which are used as stabilizers for emulsion polymerization.
U.S. Pat. Nos. 3,734,874, 3,779,993, and 4,335,220 describe the preparation of water-soluble or dispersible polyesters having anionic sulfonate groups, with no branching of the polymer chains.
U.S. Pat. No. 4,946,932 describes methods for preparing water dispersed polyester/acrylic copolymer blends via emulsion polymerization in the presence of sulfonated but unbranched polyesters as stabilizers. Similarly, U.S. Pat. Nos. 4,939,233 and 5,342,877, and WO 95/01399 describe methods for preparing water-dispersed copolymer blends via emulsion polymerization of various ethylenically unsaturated monomers using sulfonated but unbranched polyesters for the polymerization.
WO 97/28199 describes small particle size latex polymer blends comprising co-polymers of vinyl/acrylic monomers and linear chain water-dispersible sulfonate group-containing polyesters having highly specific sulfonate group contents (between 7-8 wt % of sulfonate groups in the sulfopolymer), which have low coagulum levels in the final latex.
Nevertheless, none of the references above teach or suggest the use of branched sulfopolymer stabilizers for the emulsion polymerization of ethylenically unsaturated monomers.
U.S. Pat. No. 5,277,978 describes methods for preparing water dispersible polyester/acrylic copolymer blends having coagulum contents as little as approximately 1%, via emulsion polymerization of ethylenically unsaturated monomers in the presence of sulfonated polyester stabilizers.
U.S. Pat. Nos. 5,543,488 and 5,780,151 describe the preparation of branched sulfopolyesters, and sulfopolyamides for use as adhesives, but do not teach or suggest the use of the branched sulfopolymers as stabilizers in emulsion polymerizations.
Of the above references, only U.S. Pat. No. 5,277,978 and WO 97/28199 describe achievement of coagulum levels less than approximately 1-2 wt % at sulfopolymer levels below 10 wt %. Only WO 97/28199 describes coagulum levels of less than 0.5 wt % at sulfopolymer levels below about 10 wt %.
Therefore, there is a continuing need for improved emulsion polymerization processes that provide small particle-size latexes that produce very little coagulum, while minimizing the amount of anionic emulsion stabilizers that are detrimental to the water resistance of coatings made from the latexes.